This invention relates generally to cryogenic rectification and, more particularly, to cryogenic rectification for producing xenon.
Recent medical and aerospace advances are increasing the demand for xenon. Existing methods for producing xenon, wherein the xenon is a by product of krypton production, are relatively expensive. Xenon exists in the air at a concentration of only about 0.087 ppm. In attempting to maximize rare gas production the constraints imposed often had a negative impact on the overall plant design, effectively reducing the recovery of oxygen, by requiring an additional oxygen feed to the xenon processing unit.
Accordingly it is an object of this invention to provide a system which can produce a xenon concentrate, suitable for further processing to produce high purity xenon, without imposing a production burden on the overall cryogenic air separation plant.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing xenon concentrate comprising:
(A) passing feed air into a cryogenic air separation plant having a higher pressure column and a lower pressure column, and separating the feed air by cryogenic rectification within the cryogenic air separation plant to produce oxygen-rich liquid having a xenon component;
(B) passing oxygen-rich liquid from the sump of the lower pressure column into the upper portion of a xenon concentrator column;
(C) separating the oxygen-rich liquid within the xenon concentrator column by cryogenic rectification to produce oxygen gas and xenon-richer liquid oxygen;
(D) withdrawing oxygen gas from the upper portion of the xenon concentrator column; and
(E) recovering xenon-richer liquid oxygen from the lower portion of the xenon concentrator column.
Another aspect of the invention is:
Apparatus for producing xenon concentrate comprising:
(A) a cryogenic air separation plant having a higher pressure column and a lower pressure column, and means for passing feed air into the cryogenic air separation plant;
(B) a xenon concentrator column having a bottom reboiler, and means for passing liquid from the sump of the lower pressure column into the upper portion of the xenon concentrator column;
(C) means for passing fluid to the bottom reboiler and means for withdrawing fluid from the bottom reboiler;
(D) means for withdrawing gas from the upper portion of the xenon concentrator column; and
(E) means for recovering xenon-richer liquid oxygen from the lower portion of the xenon concentrator column.
As used herein the term xe2x80x9coxygen gasxe2x80x9d means a gas having an oxygen concentration of at least 90 mole percent.
As used herein the term, xe2x80x9ccolumnxe2x80x9d, means a distillation or fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases flow countercurrently to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements. For a further discussion of distillation columns see the Chemical Engineers"" Handbook, Fifth Edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, xe2x80x9cDistillationxe2x80x9d B. D. Smith et al., page 13-3, The Continuous Distillation Process. The term, double column is used to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the volatile component(s) in the vapor phase and the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is adiabatic and includes integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out, at least in part, at temperatures at or below 150 degrees Kelvin.
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9csumpxe2x80x9d means that section of a column below the column mass transfer internals, i.e. trays or packing.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, and also containing xenon, such as ambient air.
As used herein the term xe2x80x9cxenon concentrator columnxe2x80x9d means a column which processes a feed containing xenon, and produces a xenon concentrate which has a higher xenon concentration than does the feed to the column.
As used herein the term xe2x80x9cbottom reboilerxe2x80x9d means a heat exchange device that generates column upflow vapor from column liquid.
As used herein the term xe2x80x9cproduct boilerxe2x80x9d means a heat exchanger wherein liquid from a cryogenic air separation plant, typically at increased pressure, is totally or partially vaporized by indirect heat exchange with feed air.